The federal agency that regulates the nation's pension system issued a stark warning this week about some retirement plans. The Pension Benefit Guarantee Corporation said a million workers are in plans that could be insolvent within a decade, and the people that oversee these plans say Congress needs to step in before the problem gets any worse. NPR's Jim Zarroli is here to explain the problem. And, Jim, how bad is the problem facing U.S. pension systems and what kind of workers are likely to be affected?

JIM ZARROLI, BYLINE: Well, if you listen to the Pension Benefit Guarantee Corporation, the problem is pretty dire. That is the federal agency that guarantees that workers are going to get the pension benefits they're promised if, you know, if, for instance, their company goes bankrupt. And the corporation says that the real problem is with what are called multi-employer plans, in other words, plans that a lot of different employers have paid into. More than 10 million people are in plans like these right now. Now, in the past, these kinds of plans - these multi-employer plans - were always seen as the most stable kind of pension plans because they tended to have a lot of different companies putting money into them. So, you know, if one of the companies went under, the others that remained would shoulder the burden, and workers would get paid. But in the past few decades, some of the multi-employer plans have actually become some of the most troubled.

SIEGEL: Why, why should that be? What's changed for multi-employer plans and why would they now be more likely to be at risk?

ZARROLI: Well, a lot of them are not at risk and, in fact, most of them are doing OK. But there are some that have really stumbled for various complex reasons. Two of the biggest that are in trouble right now are the Teamsters' Central States plan and the United Mine Workers of America Health and Retirement Fund. And part of the problem is that, you know, a union like the Teamsters is aging. There are too many retired workers and there aren't enough active members paying into the fun, but there've also been a lot of policy changes over the years that have made a difference. For instance, employers who were part of these big multi-employer plans have been allowed to extricate themselves from the plan by paying a fee. You know, and as more employers leave the plans then the financial condition of these plans just gets weaker and weaker and that means other employers are reluctant to join because they don't want to take on someone else's problems.

SIEGEL: Well, is Congress doing something about these plans and does anyone have any ideas about how to fix them?

ZARROLI: Well, Congress is under a lot of pressure right now to do something and, of course, there's some disagreement about what to do. The National Coordinating Committee for Multiemployer Plans, which represents the trustees of some of these plans, wants to see some big changes in the laws governing pensions. One of the things they want to do is give pension plans that are in really bad shape the ability to renegotiate cuts and retirement payments for workers. You know, as you might imagine unpopular with some of the groups that advocate for retired workers, like the Pension Rights Center, they say, you know, the deficit problem is real but it's being exaggerated. They say cutting pensions should be off the table completely. And then the pension plans come back and say, you know, if you let these plans go bankrupt then retirees will really suffer because their benefits will get cut even more. So there's a lot of disagreement right now about what to do.